The present invention relates to a process and a device for optimizing the flow rate of information passing through a multifunction cable, such as for example a cable linking a well tool to a surface installation.
When measuring and/or intervention are to be carried out in a well, tools are generally used which are lowered at the end of a multifunction cable, such as a cable known as logging cable. Such a logging cable can withstand tensile stresses and comprises several lines allowing the transmission of electrical energy to the bottomhole tools, as well as exchange of control and measuring signal between the tools and a surface installation.
The downhole tools consist, for example, of one or several sondes which may be locked in the well as a result of the opening of fastening arms under the action of hydraulic jacks. The hydraulic energy is provided by a downhole hydraulic generator supplied with electric power from a surface source through supply lines included in the cable. The surface power source is also linked to electric motors or to electromagnetic means arranged in the tools through these lines. Other lines of the cable are used for transmitting control signals to the tools and, in return, for transmitting measuring signals to a surface recording device.
Various well sondes are described in U.S. Pat. Nos. 4,428,422, 4,616,703, 4,862,425, 4,901,289.
There are numerous applications for which a large number of measuring sensors are contained in well equipment. It is notably the case in the field of seismic prospecting of wells, where an array of sensors such as geophones is used and can be distributed in a main sonde and possibly in one or several satellite sondes at various depths. The sensed signals must be transmitted to a surface recording apparatus. The number of sensors and the frequency of the signals received generally justify the transmission thereof in digitized and coded form with a high transmission rate.
The passband of the transmission lines included in the cable is generally relatively low. When the rate of the data to be transmitted becomes high enough, the limited performance of the lines acts as a brake, which make it necessary to provide the electronic systems of the tools with powerful buffer memories.
The multifunction cable which is most commonly used for petroleum applications, for example, comprises a central line, a plurality of lines arranged in rings and an outer metallic sheath.
A process for improving the rate of the data which can pass through such a cable is well-known through U.S. Pat. No. 4,855,732, this process essentially comprising transmitting between the central line and the outer sheath the coded data according to a bipolar code such as code HDB3, well-known by specialists. With such a lay-out, rates higher than 100 kilobits/second (kb/s), or even higher than 200 kb/s by optimizing the transmission factors, can be easily reached over distances of several kilometers
The present evolution of well measuring equipment makes it more and more necessary to further increase the transmission rates in order to accommodate the increase in the stream of data to be transmitted towards the surface installations.
A conventional solution used in the telecommunication field would consist in utilizing cables with a wider passband. This is not possible for transmissions in wells where a standard logging cable generally available on the site has to be used.
SUMMARY OF THE INVENTION
The process according to the invention makes it possible to avoid the limitations imposed by the existing multifunction cables (logging cables) and to optimize the transmission rate of signals on transmission lines included in said cables in order to obtain the high flow rates necessary for transmitting the volumes of data collected by the sensors in the most recent well equipment, without modifying the allowable transmission error rate.
The process comprises :
coding the digitized data to be transmitted through voltages selected from a set of coding voltages comprising at least two voltages of determined constant levels, and PA0 widening towards high frequencies the passband of each transmission line used by combination with correcting circuits selected so that the transfer function of the corrected line is substantially that of a reference filter, such as a filter of the Bessel type, in a frequency interval on either side of the 3-dB cut-off frequency (fc) of the corrected line, the frequency of the upper bound of said interval being proportional to this cut-off frequency, the proportionality coefficient (k.sub.1) being higher than 2 and said cut-off frequency fc of the corrected line being selected as a function of the number of coding voltages of said set and of the allowable error rate.
The correcting circuits are, for example, selected in such a way that the transfer function of each corrected line substantially corresponds to that of a Bessel filter, at least in a frequency interval (k.sub.2 fc and k.sub.1 fc), where k.sub.1 is a multiplicative factor at least equal to 2.5 and k.sub.2 is a multiplicative factor of the order of 0.2.
According to one embodiment, transmission circuits are selected adapted for applying to the signals in said frequency interval a gain at the most equal to a limiting value decreasing correlatively with an increase in the selected number of coding voltages, for a set error rate depending on the noise level restored through said correcting circuits associated with each transmission line, and said cut-off frequency (fc), which the maximum transmission frequency depends on, is selected so that the amplitude increase to be applied to the signals transmitted on each transmission line is at the most equal to said limiting value (G).
According to a preferred embodiment, the transmission rate is increased by coding the signals to be transmitted by means of a set of coding voltages comprising at least eight coding voltages.
The transmission rate can be increased further by advantageously selecting a set of coding voltages comprising sixteen coding voltages.
This multiple voltage coding, this addition of particular filtering circuits, and this controlled widening of the passband make it possible to multiply by 3 or 4 the possible transmission rate of the most common multifunction cables, and this without increasing the transmission error rate.
The device for implementing the process comprises, for example, a coding set adapted for coding digitized signals to be transmitted with a selected number of coding voltages and for applying the coded digitized signals onto a transmission line, and filtering circuits adapted in such a way that the line combined with said filtering circuits has a transfer function preferably similar to that of a Bessel filter with a cut-off frequency fc selected as a function of the number of coding voltages used, of the transfer function of the transmission line alone, and of the noise features of said filtering circuits, in a frequency interval extending on either side of said cut-off frequency fc.